A radiator and a thermostat are disposed in a cooling channel through which a coolant for an engine circulates. If the temperature of the coolant is equal to or higher than a predetermined temperature, the thermostat opens a valve to circulate the coolant through the radiator to dissipate heat; otherwise, if the temperature of the coolant is lower than the predetermined temperature, the thermostat closes the valve to prevent the coolant from circulating through the radiator (i.e., causes the coolant to bypass the radiator), thereby raising the temperature.
In the event of a valve-open failure (i.e., a failure of the valve being fixed in an open state) in the thermostat provided in such an engine cooling system, even a coolant at a temperature under the predetermined temperature also circulates the radiator. This hinders a quick increase in the coolant temperature to cause time-consuming warm-up of the engine, resulting in poor fuel economy and an increase in emission.
Techniques for determining a valve-open failure in a thermostat have accordingly been proposed. For example, an apparatus described in Patent Literature 1 (Japanese Patent No. 4661767) compares an estimated temperature of a coolant for an engine and a detected temperature with a reference temperature to determine the operational state of the thermostat. The apparatus also cancels the determined results during a vehicle operation condition that may cause a wrong determination, in order to increase the accuracy of determination.
The temperature of a coolant for an engine, which may be estimated from the amounts of heat absorbed and heat dissipated by the coolant, needs to be estimated by arithmetic operations different between the engine operating mode and the engine stop mode since heat-absorption and heat-dissipation by the coolant are different between these two modes. Nevertheless, estimation by the arithmetic expressions different between the engine operating mode and the engine stop mode leads to an increase in an operation load. A single arithmetic expression for different operation modes, however, lowers the accuracy of estimating the temperature of the coolant in the case requiring different arithmetic expressions.
In addition, a vehicle having the function of brief stopping without idling experiences a longer engine-stop time (including the time period of the engine being automatically stopped) than that of a vehicle without this function. For this reason, a single arithmetic expression for both the engine operating mode and the engine stop mode may lead to reduced estimation accuracy of the temperature of the coolant, resulting in an erroneous decision in a determination of a thermostat failure. Furthermore, vehicles that can run while their engines being automatically stopped for a long time, such as plug-in hybrid electric vehicles (PHEVs), have been put into practical use recently, which might increase erroneous decision cases in a failure determination.
The apparatus disclosed in Patent Literature 1 cancels the results of a failure determination determined within a time period from a brief stop without idling to a lapse of a predetermined time from the restart of the engine because this period for a vehicle tends to cause an erroneous decision. Such cancelling, notwithstanding, gives no results on the normality of the thermostat and requires another failure determination to acquire accurate results, which, unfortunately, hinders immediate determination of the operational state of the thermostat. In particular, a vehicle capable of running while its engine being automatically stopped for a long while disadvantageously leads to significantly lower accuracy of estimating a coolant temperature, which phenomenon needs a longer predetermined time after engine restarts to prevent an erroneous determination. As a result, cancelling of the determined results increases. Consequently, acquiring determined results takes a long time.